In the proposed research we plan to utilize the anti-DNP murine myeloma system to determine the ultrastructural localization of the cell surface associated immunoglobulin of plasma cells. The anti-DNP myeloma makes available large amounts of purified, well-characterized and highly specific reagents. When these reagents are conjugated with ferritin and applied in freeze-etch electron microscopic studies they become powerful cell surface membrane probes. We describe a series of studies which are designed to: 1) examine, for the first time, the ultrastructural location of plasmacyte surface immunoglobulin and, 2) examine the ultrastructural localization of the surface immunoglobulin when it functions as a tumor-specific transplantation antigen. These studies may contribute to a further understanding of cell membrane organization, and may provide an insight to the paradoxical growth of tumor cells in specifically immune hosts.